1. Field of the Invention
The present invention relates to a process for producing iron ore pellets and, more particularly, to a process for producing iron ore pellets from raw materials containing iron ore with a high content of crystal water.
2. Description of the Prior Art
Production of iron ore pellets by the grate kiln system involves the steps of forming spherical green pellets (9.5-11 mm in diameter), drying and preheating them (in layers about 30 cm thick) in the travelling grate to impart prescribed strength to them, firing the preheated pellets at 1250-1350.degree. C. in the firing rotary kiln, and finally cooling the fired pellets in the cooler. In this way there are obtained iron ore pellets to be used in the blast furnace.
For the efficient production of high-quality pellets by the grate kiln system mentioned above, it is necessary to charge the rotary kiln with preheated pellets which have been given prescribed crushing strength by complete heat treatment in the travelling grate. The travelling grate will be operated under different conditions depending on the kind of iron ore; therefore, the productivity and quality are variable. For example, the operating time would be shorter and the preheating temperature would be higher for magnetite than for hematite (because the former generates heat by oxidation at 700.degree. C. and above).
Usually, the travelling grate consists of three zones--drying, dehydrating, and preheating zone. The drying zone is designed to remove water from green pellets at 180-250.degree. C., the dehydrating zone is designed to remove 1-3% of crystal water at 250-400.degree. C., and the preheating zone is designed to give pellets crushing strength sufficient for them to withstand tumbling firing in the rotary kiln at about 1000.degree. C. In this way, preheated pellets are produced.
The continued high steel production has led to the mining of iron ore at deeper deposits than before. Iron ore mined at deep deposits contains more crystal water. Such iron ore poses a problem when it is made into pellets by the grate kiln. In other words, particles of iron ore with a high content of crystal water shrink when green pellets are dehydrated and preheated. As the result, preheated pellets increase in porosity, with reduced bonding between particles, and decrease in strength. In addition, removing crystal water needs a large amount of reaction heat, which leads to a decrease in pellet temperature. To compensate for this, it is necessary to supply additional heat energy for preheating and firing.
Preheated pellets with reduced strength largely become powder during firing in the rotary kiln, which leads to low yields and gives rise to kiln rings.
Several methods have been proposed as follows to improve the strength of preheated pellets.
(a) Raising the preheating temperature and/or prolonging the preheating time for green pellets. This decreases the pellet porosity and promotes bonding between iron ore particles. PA1 (b) Incorporating iron ore powder with finely divided bentonite. This promotes the granulation of green pellets, thereby decreasing the pellet porosity. PA1 (c) Incorporating iron ore powder with an organic binder such as cellulose. This promotes the granulation of green pellets, thereby decreasing the pellet porosity.
Unfortunately, method (a) suffers the disadvantage of requiring renovation of production facilities for increased heat resistance and also requiring additional fuel for preheating, which leads to a cost increase. Method (b) suffers the disadvantage of requiring a large amount of bentonite powder, which leads to a cost increase and degraded pellets. Method (c) also suffers the disadvantage of requiring a large amount of organic binder, which not only increases production cost but also deteriorates the strength of pellets because any organic binder burns to form voids during preheating and firing, thereby increasing porosity.